1. Field of the Invention
The present invention relates to wireless communications systems and, more specifically, to a system for data communications among a plurality of terminals which are connected to one another through a wireless network, and a method used therein for scheduling (method for assigning a transmission band for data transmission for each communications link), dividing a data packet, and carrying out re-transmission when an error occurs.
2. Description of the Background Art
There have been various systems for data communications among personal computers (hereinafter, PCs) or between PCs and a host computer connected to one another through a network. A wired network such as Ethernet of IEEE 802.3 and Token-Ring of IEEE 802.5 have been part of the mainstream of such a network. With the advent of portable PC terminals which do not require any wiring, however, such a wired network is gradually shifting to a wireless network.
The CMSA (Carrier Sense Multiple Access) method, which is one of the conventional access techniques using a wireless network, will now be briefly described with reference to FIG. 21.
In FIG. 21, PCs 501 to 504 are connected to wireless access devices 505 to 508, respectively. The wireless access devices 505 to 508 are connected to one another through a wireless network. Here, a case is described where data is transmitted from the PC 501 to the PC 502, and then, after a short while, other data is transmitted from the PC 503 to PC 504.
When receiving a data transmission request from the PC 501, the wireless access device 505 measures received field intensity through a receiving operation so as to check whether or not any other wireless access device is in communication. If not, the wireless access device 505 sequentially transmits the data that is provided by the PC 501 to the wireless access device 506. The wireless access device 506 transfers the data that is received from the wireless access device 505 to the PC 502.
When receiving a data transmission request from the PC 503 shortly after the wireless device 505 receives the above-mentioned request from the PC 501, the wireless access device 507 similarly measures received field intensity through the receiving operation so as to check whether or not any other wireless access device is in communication. At this time, the wireless access device 505 is still carrying out data transmission. Therefore, the wireless access device 507 waits until this data transmission ends. Then, after this data transmission ends, the wireless access device 507 sequentially transmits the data that is provided by the PC 503 to the wireless access device 508. The wireless access device 508 transfers the data that is received from the wireless access device 507 to the PC 504.
However, in conventional wireless communications systems using the CSMA method, if the plurality of PCs each send a transmission request simultaneously, the wireless access devices try to start transmission, leading to transmission collisions on the wireless network. This is because the wireless access devices can only check whether transmission is actually being made at the time of measuring field intensity. In other words, the wireless access devices cannot detect transmission that will be made in future. Moreover, the wireless access devices are not capable of detecting collision, and therefore, the wireless access devices may erroneously determine that transmission has succeeded even though transmission has not succeeded due to collision. Such collision will occur more frequently as the number of wireless access devices or the number of transmissions that are tried by the wireless access devices increases.
To get around the above problem, time-division transmission methods such as TDM and TDMA can be applied so as to avoid transmission collisions on the wireless network when a plurality of wireless access devices each send a data transmission request simultaneously. In such time-division methods, a transmission band is previously divided into several bands, and a divided band is assigned for each data transmission request. Thus, each wireless access device can make data transmission by using each specific transmission band, and transmission collisions can be prevented on the wireless network.
In the near future, a network such as LAN will be introduced to households. For such a household network, a wireless network is desirable because wiring is not required and devices which are connected to the network can be easily moved. With such a network, digital video data will mostly be used as transmission contents. However, such digital video data is generally large in volume, and requires high speed for transmission. Moreover, for the transmission of digital video data from a set-top box or video player to a television set, a real-time transmission is required.
In the above-described conventional wireless communications system using the CSMA method, however, transmission efficiency is low because transmission collisions on the wireless network should be avoided. Therefore, it is difficult to achieve real-time, high-capacity, high-speed transmission of digital video data.
Furthermore, in the conventional time-division wireless communications system, a transmission band is fixedly assigned to each generated data transmission request. Therefore, when a state of data transmission is changed, the transmission band cannot be changed until the data transmission ends. One example of wireless communications systems where a transmission band is fixedly assigned is disclosed in Japanese Patent Laid-Open Publication No. 11-252663 (1999-252663). Therefore, in the conventional wireless communications system, real-time data transmission may be impaired.